Most bivalve hitchickers usually don't survive long but phytoplankton will help. DT's or Biomarine Liquid Life. In the beginning at least, you shouldn't need anything. There should be enough nutrient from the rock to keep everything fed.
Once fish are added, what you feed them will typically be enough to feed everything else at the same time (species depending). Don't get caught up in buying foods for every animal you might find. Secondary feedings from fish food and waste are often enough for everything else as well.